in fact, the premise of the question is not right

"add(1)(2);
// returns 3"

No. It is not. This returns a function.

In the tests, it is the == operator that does the conversion, as you can read it here:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Equality

"If the operands are of different types, try to convert them to the same type before comparing:
When comparing a number to a string, try to convert the string to a numeric value.

• If one of the operands is Boolean, convert the Boolean operand to 1 if it is true and +0 if it is false.
• If one of the operands is an object and the other is a number or a string, try to convert the object to a primitive using the object's valueOf() and toString() methods."

I was heading towards this solution, but I didn't finish...

Great clever work!

Great joke though!